This invention relates to methods for generating doubled haploid plants from microspores, and to doubled haploid plants produced by the methods disclosed herein.
Although plant breeding programs worldwide have made considerable progress developing new cultivars with improved disease resistances, yields and other, useful traits, breeding as a whole relies on screening numerous plants to identify novel, desirable characteristics. Very large numbers of progeny from crosses often must be grown and evaluated over several years in order to select one or a few plants with a desired combination of traits.
In a typical plant breeding experiment, two parent plants are crossed and the resulting progeny (the F1 generation) are screened and a plant (termed the F1 plant) identified that possesses a desirable combination of phenotypic traits. The F1 plant is then self-fertilized to yield a population of progeny plants (termed F2 plants) that must be individually analyzed to determine which F2 plants possess the desired combination of phenotypic traits originally introduced in the F1 plant. If, as is often the case, the desired phenotypic traits derive from the combined effect of several genes, then the number of F2 progeny plants that must be screened depends on the number of genetic differences between the parents of the F1 plant. Thus, the greater the number of genetically-controlled differences between parents of the F1 plant, the larger the number of F2 progeny that must be grown and evaluated, and the lower the probability of obtaining progeny with all the desired traits.
For example, if the two parents of the F1 plant differ by 25 gene alleles (not an unusually great number in breeding), more land than exists on the earth would be needed to grow all possible genotype combinations which can occur in the F2 generation derived from the self-fertilized F1 plant (Konzak, C. F. et al. In: Elliott, L. (ed.) STEEPxe2x80x94Conservation Concepts and Accomplishment, pp. 247-273, 1987. ). Further, once an F2 plant has been identified that exhibits the same, desirable, phenotypic trait(s) as the cross parents, the process of self-fertilization and analysis of the resulting progeny must be repeated several times until a homozygous population of plants is obtained which breed true for the desired phenotypic character, i.e., all progeny derived from the true-breeding population exhibit the desired, phenotypic trait (though the progeny may not be true-breeding for unselected traits).
One possible solution to the problem of screening large numbers of progeny is to produce them from the gametic cells as haploid plants, the chromosomes of which can be doubled using colchicine or other means to achieve instantly homozygous, doubled-haploid plants. In particular, doubled haploids can be produced from the microspores which normally give rise to pollen grains.
The life cycle of flowering plants exhibits an alteration of generations between a sporophytic (diploid) phase and a gametophytic (haploid) phase. Meiosis produces the first cells of the haploid generation which are either microspores (male) or megaspores (female). Microspores divide and develop within anthers to become mature male gametophytes (pollen). In normal development, microspores are genetically programmed for terminal differentiation to form mature pollen through two cell divisions. However, under certain conditions, microspores can be induced to initiate sporophytic development which leads to the formation of haploid or doubled haploid xe2x80x9cembryoidsxe2x80x9d. These embryoids can give rise to mature plants, that are either haploids or doubled haploids, through subsequent sporophytic development. The process by which plants are produced from microspores is termed pollen-embryogenesis or androgenesis, i.e., from the male gametophyte. Androgenesis is of significant interest for developmental genetic research as well as plant breeding and biotechnology, since it is a means to produce genetically true-breeding, doubled haploid plants.
As shown in Table 1, by producing doubled-haploid (also termed polyhaploid) progeny, the number of possible gene combinations for any number of inherited traits is more manageable.
Thus, marked improvements in the economics of breeding can be achieved via doubled haploid production, since selection and other procedural efficiencies can be markedly improved by using true-breeding (homozygous) progenies. With doubled haploid production systems, homozygosity is achieved in one generation. Thus, the breeder can eliminate the numerous cycles of inbreeding necessary by conventional methods to achieve practical levels of homozygosity. Indeed, true homozygosity for all traits is not even achievable by conventional breeding methods. Consequently, an efficient doubled haploid technology would enable breeders to reduce the time and the cost of cultivar development relative to conventional breeding practices.
Thus, there is a need for a method of efficiently producing doubled haploid plants that is applicable to a wide variety of plant species.
In accordance with the foregoing, in one aspect the present invention provides methods of generating doubled haploid and/or haploid plants from microspores.
The methods of the present invention for producing plants from microspores include the steps of: selecting plant material including microspores at a developmental stage amenable to androgenic induction; subjecting the microspores to temperature stress to obtain stressed microspores; contacting the microspores with an amount of a sporophytic development inducer effective to induce sporophytic development and chromosome doubling, the contacting step occurring before, during, after, or overlapping with any portion of the temperature stress step; isolating the stressed microspores; and coculturing the isolated microspores with either ovary-conditioned medium or at least one live plant ovary. Preferably, microspores are subjected to nutrient stress at the same time that they are subjected to temperature stress. Preferably, microspores are contacted with an amount of an auxin and/or a cell spindle inhibiting agent before, during, after, or overlapping with any portion of the temperature stress step.
In the practice of the methods of the present invention, plant material is selected that bears reproductive organs containing microspores at a developmental stage that is amenable to androgenic induction. Preferably the selected plant material is tillers or branches bearing spikes or flowers that contain microspores in the mid uninucleate to early binucleate stages of development. The microspores are treated by contacting the selected plant material with an aqueous medium, such as water, and subjecting the selected plant material to temperature stress, and optionally to nutrient stress. Temperature stress is effected by incubating the selected plant material, in contact with aqueous medium, at a preferred temperature of from about 4xc2x0 C. to about 40xc2x0 C., more preferably from about 28xc2x0 C. to about 35xc2x0 C., most preferably at about 33xc2x0 C., for a period of from about half an hour to about 72 hours. Nutrient stress is effected by utilizing, in the aqueous medium, an amount of at least one nutrient (such as nitrogen, calcium, phosphorus or sulfur) that is less than the amount of that nutrient necessary for the optimal growth and development of the microspores. Preferably nutrient stress is effected by utilizing water as the aqueous medium. Most preferably nutrient stress is effected by utilizing diluted NPB 98 as the aqueous medium, preferably NPB 98 medium diluted with an amount of water sufficient to dilute NPB 98 to less than or equal to 80% of its undiluted concentration. The selected plant material is also contacted with an effective amount of at least one sporophytic development inducer (as further described herein), such as 2-hydroxynicotinic acid (2-HNA), violuric acid, 2-hydroxyproline or ethrel. Preferably the selected plant material is contacted with an effective amount of a sporophytic development inducer and an effective amount of an auxin (preferably 2,4-dichlorophenoxyacetic acid) and/or an effective amount of a cell spindle inhibiting agent (such as pronamide). The presently preferred concentration range for auxin is from about 0.1 mg/l to about 25 mg/l, more preferably from about 0.2 mg/l to about 10.0 mg/l, most preferably from about 0.5 mg/l to about 4.0 mg/l. The presently preferred concentration range for sporophytic development inducer is from about 0.001 mg/l to about 1000 mg/l. The presently most preferred concentration range for sporophytic development inducer is from about 1 mg/l to about 500 mg/l. The presently preferred concentration range for cell spindle inhibiting agent is from about 1.0 xcexcM to about 200 xcexcM.
Optionally, the selected plant material is contacted with an effective amount of a cytokinin, preferably kinetin or BAP, and/or an effective amount of a gibberellin. The preferred concentration range for cytokin is from about 0.1 mg/l to about 10 mg/l, more preferably from about 0.2 mg/l to about 4.0 mg/l, most preferably from about 0.5 mg/l to about 2.0 mg/l. The presently preferred concentration range for gibberellin is from about 0.01 mg/l to about 20 mg/l, most preferably from about 0.2 mg/l to about 4.0 mg/l. The selected plant material is contacted with some or all of the foregoing chemical agents (sporophytic development inducer, cell spindle inhibiting agent, auxin, cytokinin and/or gibberellin) before, during, after, or overlapping with any portion of the temperature stress treatment. The treated microspores are isolated preferably by macerating the selected, treated plant tissue, filtering the macerated plant tissue and subjecting the filtrate to density centrifugation, preferably utilizing a solution of percoll, ficoll or mannitol, most preferably a 0.3 M mannitol solution, layered over a higher density solution of percoll, ficoll, polyethylene glycol or a sugar, preferably maltose, most preferably 0.58 M maltose. The isolated, treated microspores are then cultured in a liquid nutrient suspension medium, such as medium NPB98 or NPB 99, preferably NPB 99, supplemented with either plant ovary conditioned medium or at least one live plant ovary, until the microspores develop into embryoids. Preferably the plant ovaries (including the ovaries used to prepare plant ovary conditioned medium) are obtained from wheat varieties xe2x80x9cChrisxe2x80x9d or xe2x80x9cPavon 76xe2x80x9d, but ovaries from a wide range of genotypes, including Igri barley, are effective. The embryoids are transferred to a regeneration medium and incubated therein until the embryoids develop into plants. The resulting plants may be doubled haploids, or they may be haploids which can be converted to doubled haploids by treatment with a chromosome doubling agent such as colchicine. It will be understood, however, that the microspores can be isolated before being contacted with an aqueous medium and being subjected to temperature stress.
The methods of the present invention for producing plants from microspores may optionally include the step of genetically transforming the microspores. Microspores can be genetically transformed at any time during treatment of the microspores in accordance with the methods of the present invention. The presently preferred methods of genetically transforming microspores are biolistic gene transfer utilizing a particle gun or electroporation of plasmolyzed microspores. Thus, in one aspect, the present invention provides genetically transformed plants regenerated from microspores.
In other aspects, the present invention provides methods of initiating microspore embryogenesis including the steps of: selecting plant material including microspores at a developmental stage amenable to androgenic induction; subjecting the microspores to temperature stress to obtain stressed microspores; and contacting the microspores with an amount of a sporophytic development inducer effective to induce sporophytic development and chromosome doubling, the contacting step occurring before, during, after, or overlapping with any portion of the temperature stress step. Preferably, microspores are subjected to nutrient stress at the same time that they are subjected to temperature stress. Preferably, microspores are contacted with an amount of an auxin and/or a cell spindle inhibiting agent before, during, after, or overlapping with any portion of the temperature stress step.
In the practice of the methods of the present invention for initiating microspore embryogenesis, plant material is selected that bears reproductive organs containing microspores at a developmental stage that is amenable to androgenic induction. Preferably the selected plant material is tillers or branches bearing spikes or flowers that contain microspores in the mid uninucleate to early binucleate stages of development. The microspores are treated by contacting the selected plant material with an aqueous medium, such as water, and subjecting the selected plant material to temperature stress, and optionally to nutrient stress. Temperature stress is effected by incubating the selected plant material, in contact with aqueous medium, at a preferred temperature of from about 4xc2x0 C. to about 40xc2x0 C., more preferably from about 28xc2x0 C. to about 35xc2x0 C., most preferably at about 33xc2x0 C., for a period of from about half an hour to about 72 hours. Nutrient stress is effected by utilizing, in the aqueous medium, an amount of at least one nutrient that is less than the amount of that nutrient necessary for the optimal growth and development of the microspores. Preferably nutrient stress is effected by utilizing water as the aqueous medium. Most preferably nutrient stress is effected by utilizing diluted NPB 98 as the aqueous medium, preferably NPB 98 medium diluted with an amount of water sufficient to dilute NPB 98 to less than or equal to 80% of its undiluted concentration. The selected plant material is preferably subjected to nutrient stress for a period of from about half an hour to about ninety six hours, more preferably from about half an hour to about seventy two hours. The selected plant material is also contacted with an effective amount of at least one sporophytic development inducer (as further described herein), such as 2-hydroxynicotinic acid (2-HNA), violuric acid, 2-hydroxyproline or ethrel. Preferably the selected plant material is contacted with an effective amount of a sporophytic development inducer and an effective amount of an auxin (preferably 2,4-dichlorophenoxyacetic acid) and/or an effective amount of a cell spindle inhibiting agent (such as pronamide). The presently preferred concentration range for auxin is from about 0.1 mg/l to about 25 mg/l, more preferably from about 0.2 mg/l to about 10.0 mg/l, most preferably from about 0.5 mg/l to about 4.0 mg/l. The presently preferred concentration range for sporophytic development inducer is from about 0.001 mg/l to about 1000 mg/l. The presently most preferred concentration range for sporophytic development inducer is from about 1 mg/l to about 500 mg/l. The presently preferred concentration range for cell spindle inhibiting agent is from about 1.0 xcexcM to about 200 xcexcM.
Optionally, the selected plant material is contacted with an effective amount of a cytokinin, preferably kinetin or BAP, and/or an effective amount of a gibberellin. The preferred concentration range for cytokinin is from about 0.1 mg/l to about 10 mg/l, more preferably from about 0.2 mg/l to about 4.0 mg/l, most preferably from about 0.5 mg/l to about 2.0 mg/l. The presently preferred concentration range for gibberellin is from about 0.01 mg/l to about 20 mg/l, most preferably from about 0.2 mg/l to about 4.0 mg/l. The selected plant material is contacted with some or all of the foregoing chemical agents (sporophytic development inducer, cell spindle inhibiting agent, auxin, cytokinin and/or gibberellin) before, during, after, or overlapping with any portion of the temperature stress treatment. It is understood, however, that the microspores can be isolated before being contacted with an aqueous medium and being subjected to temperature stress.
The methods of the present invention for initiating microspore embryogenesis may optionally include the step of genetically transforming the microspores. Preferably uninucleate microspores are used for genetic transformation. Microspores can be genetically transformed at any time during treatment of the microspores in accordance with the methods of the present invention. The presently preferred methods of genetically transforming microspores are biolistic gene transfer utilizing a particle gun or electroporation of plasmolyzed microspores.
In another aspect of the invention, methods are provided for stimulating susceptible plant microspores to form embryoids. The methods include the step of incubating susceptible microspores with at least one whole plant ovary, or with plant ovary conditioned medium capable of stimulating susceptible plant microspores to form embryoids.
In another aspect of the present invention, doubled haploid and/or haploid plants are provided that are produced according to the methods of the present invention.
The term doubled haploid (abbreviated as DH) is used herein to refer to plants produced by doubling the chromosome number of a gamete-derived haploid plant which is produced via male gamete sporophytic divisions. The chromosome doubling (including spontaneous chromosome doubling) can occur at any stage in the process of converting a microspore to a whole plant, or can be induced, for example, by treating haploid plants with colchicine.
The term xe2x80x9cmicrosporexe2x80x9d refers herein to the male gametophyte of a plant, including all stages of development from meiosis through formation of the mature pollen grain.
The term xe2x80x9candrogenic inductionxe2x80x9d means induction of androgenesis, i.e., the process by which plants are produced from microspores.
The abbreviation mg/l means milligrams per liter.
The methods of the present invention are applicable to a broad range of plant species, including dicotyledonous plants and monocotyledonous plants. Representative examples of plants which can be treated in accordance with the methods of the present invention include, but are not limited to: wheat, barley, rice, corn, triticale, rye, millet, rice, flax, wheat grasses (for example, Agropyron, Elytrigia), pasture grasses, rye, orchard and brome grasses (e.g., Lolium spp.), turf grasses (e.g., Poa pratensis), alfalfa, clover, soybeans, peanuts, ornamentals including garden and commercial flower and bulb species, fruit trees and nut trees, vegetable species (e.g., cucurbits, onions, tomato, carrot, potato and other solanaceous plants, beans, peas, lentils), Brassica species such as oil seed rape, as well as interspecies hybrids (e.g., triploids, pentaploids, tetraploids, hexaploids, septaploids). The methods of the present invention can be used, for example, to produce inbred lines for use in hybrid seed production, especially to generate vigorous inbred lines from hybrids of crop plants that are susceptible to inbreeding depression. Additionally, the methods of the present invention can be used, for example, to obtain variable progeny from pollen-producing apomictic species, such as blue grasses (Poa spp.) and buffalo grasses (Buchloe spp.).
Preferably, the methods of the present invention utilize wheat spikes as starting material. The methods of the present invention have been successfully applied to spikes from a range of wheat genotypes including, but not limited to: the spring wheats, Calorwa, Chris, Pavon 76, Penawawa, Spiltman, Waldron, WED 202-16-2, Wawawai, SWSW96005, and winter wheats Claire, Eltan, Platte, Enola, Soisson, BonPain, Madsen and Svilena. The presently preferred wheat genotypes are Chris, WED 202-16-2 and Pavon 76. Spring wheat cv. Pavon 76 is generally considered to be a highly androgenic genotype based upon results from anther culture, although it produces a high proportion of albino plants, and a relatively low frequency of doubled haploid plants. The androgenic microspores of both Chris and WED 202-16-2 produce high numbers of green progeny, many of which are spontaneously DH. The presently most preferred wheat cultivar is Chris. Chris is a public variety and Pavon 76 is available from the USDA Cereals Collection, 1691S 2700W, Aberdeen ID 83210. WED 202-16-2 is a T. dicoccoides/Pavon 76 derivative from the Volcani Institute, Bet Degan, Israel.
The methods of the present invention permit the production of doubled haploid plants from wheat varieties and cultivars that have long been considered recalcitrant or non-responsive to anther or microspore culture. For example, Waldron is a spring wheat considered recalcitrant to established anther culture methods. Nonetheless, Waldron is responsive to the methods of the present invention.
In many cases, wheat cultivars that are recalcitrant to other methods, such as Waldron and WPB926, can be induced to begin sporophytic divisions at a high frequency utilizing the methods of the present invention. Microspore cell divisions of some of the less responsive genotypes are arrested prior to the emergence of a multi-nucleate pro-embryoid from a common microspore cell wall. A solution to this problem provided by the present invention is to treat plant material containing microspores with novel culture media NPB 98 or NPB 99, the compositions of which are set forth in Table 6.
An alternative solution to the problem of arrested microspore cell divisions is to make crosses between the recalcitrant cultivars, such as Waldron or WPB 926, and cultivars that efficiently produce green plants from embryoids, such as Chris and WED 202-16-2. In this approach, the methods of the present invention can be incorporated into a more general plant breeding program in which genotypes that are amenable to culture according to the methods of the present invention are crossed with less amenable genotypes which have other, desirable characteristics. For example, Pavon 76, which produces many embryoids, but relatively few green plants when treated in accordance with the methods of the present invention, can be crossed with WED 202-16-2, Chris, or any other wheat genotype which produces a high frequency of green plants from embryoids when treated in accordance with the methods of the present invention. The resulting doubled haploid progeny can be screened for those genotypes that produce many embryoids and many green plants. Similarly, crossing Chris, Pavon 76 or WED 202-16-2 with a recalcitrant genotype will result in progeny that are amenable to culture according to the methods of the present invention, and which also possess the desirable traits of the recalcitrant parent. The strategy of crossing a genotype that is amenable to the production of green, double haploid plants with a more recalcitrant plant species, having some other desirable trait(s), is generally applicable to any plant species.
The recalcitrant genotypes especially respond to culture in NPB 98 or NPB 99 that is conditioned by the prior growth of plant ovaries at a density of 3 to 4 ovaries per milliliter of medium for 7 to 14 days prior to use in microspore culture. Media so conditioned have been shown to accelerate cell divisions and embryogenic development of cultured micrspores. Typically, mature embryoids can be obtained one week earlier than in cultures that utilize whole ovaries instead of ovary-conditioned medium.
Wheat plants that are used to provide the microspore starting material (referred to as donor plants) in the practice of the presently preferred embodiment of the present invention may be cultivated in the field, but preferably are cultivated in an artificial, relatively sterile environment, such as a greenhouse. Field-grown wheat plants are often heavily infested with microorganisms that contaminate all stages of the microspore embryogenic process, unless an effective disinfectant treatment is used. For example, the starting plant material used in the methods of the present invention can be treated with a 20% (v/v) solution of commercial hypochlorite or chlorine bleach. Any standard growth regime that is known to one of ordinary skill in the art for growing wheat, preferably in a greenhouse, can be utilized in the practice of the present invention.
In the presently preferred embodiment of the present invention, fresh wheat tillers are cut below the top two nodes. The use of whole tillers avoids the difficulty and inconvenience associated with isolating anthers prior to treatment. The selected tillers should contain microspores at the appropriate stage of development. In general, developing microspores that have at least completed meiosis are useful in the practice of the present invention. Preferably, microspores enclosed within the anthers in the middle section of a spike should be in the mid uninucleate to early binucleate stages, most preferably in the late uninucleate stage, of development. Morphological features of tillers containing microspores at these stages can easily be established for each plant variety by comparing the morphology of the tiller with the microspore developmental stage as determined by microscopic examination with acetocarmine stain. The stages of microspore development are set forth in Bennett, M. D. et al., Philosophical Transactions of the Royal Society (Lond.), B issue, 266:39-81 (1973), which is incorporated herein by reference. The morphology of a wheat tiller is set forth in the following publications, each of which is incorporated herein by reference: Percival, J., The Wheat Plant, A Monograph, E. P. Dutton and Co., New York (1921); Montana State University Bulletin 4387, and University of Idaho Series 118.
Excised tillers are subjected to stress treatment, such as nutrient and/or temperature stress, treatment with sporophytic development inducers, and optionally to treatment with auxins, cell spindle inhibiting agents, cytokinins and gibberellins as is herein described. In one representative embodiment, leaves are removed from the selected tillers, and the tillers are placed in a flask, preferably an Ehrlenmeyer flask, containing sterile, distilled water. Several tillers may be placed in the same flask. The flask containing the tillers is placed inside a thin plastic bag, which is placed in an incubator at the appropriate temperature for stress treatment. Alternatively, the flask containing tillers can be stored in a refrigerator at 4xc2x0 C. for up to 1 month before subjecting the tillers to temperature stress treatment. Temperature stress is carried out preferably from between about 4xc2x0 C. to about 40xc2x0 C. The optimum period of pretreatment varies with the genotype, but is preferably from between about 0.5 hours and about 72 hours. Also, in general, the higher the temperature used to stress the plant material, the shorter will be the time required to temperature stress the plant material. Alternatively, microwave radiation (having an energy of 10xe2x88x926 to 10xe2x88x923 eV) can be used to subject the microspores to temperature stress. Nutrient stress is effected by utilizing, in the aqueous medium, an amount of at least one nutrient that is less than the amount of that nutrient necessary for the optimal growth and development of the microspores in the aqueous medium. Preferably nutrient stress is effected by utilizing water as the aqueous medium. Most preferably nutrient stress is effected by utilizing diluted NPB 98 as the aqueous medium, preferably NPB 98 medium diluted with an amount of water sufficient to dilute NPB 98 to less than or equal to 80% of its undiluted concentration. Nutrient stress is one way in which to promote the induction of sporophytic development from microspores and can be used, for example, when dealing with microspores from plant genotypes that are resistant to androgenic induction.
It is important that after stress treatment, the tillers should not be stored in a refrigerator at 4xc2x0 C. because the microspore viability decreases sharply. After the stress treatment, plant material should be processed as soon as possible. The flask with tillers is removed from the incubator after temperature stress and the spikelets from each spike are excised and the microspores isolated as described more fully herein.
In order to increase the yields of androgenic microspores induced by the stress treatments, the microspores in the tillers are contacted with an effective amount of a sporophytic development inducer. Most preferably, the microspores in the tillers are contacted with an effective amount of a sporophytic development inducer and an effective amount of an auxin and/or a cell spindle inhibiting agent. Optionally the microspores in the tillers are contacted with an effective amount of a cytokinin and/or a gibberellin. The microspores are contacted with the foregoing chemical agents before, during, after, or overlapping with any portion of the temperature stress treatment. Preferably the microspores are contacted with the foregoing chemical agents before the temperature stress treatment.
Sporophytic development inducers useful in the practice of the present invention induce plant microspores to switch from gametophytic development to sporophytic development. By way of non-limiting example, sporophytic development inducers useful in the practice of the present invention may cause the development of inviable pollen grains, multicellular or multinucleate pollen grains, arrest starch formation in developing microspores and cause physical deformation of mature pollen grains that develop from microspores treated with a sporophytic development inducer. Many sporophytic development inducers useful in the practice of the present invention are chemical hybridizing agents. Chemical hybridizing agents (abbreviated as CHAs) are chemicals which when applied to plants cause the plants to produce inviable pollen. Other sporophytic development inducers useful in the present invention include, but are not limited to: amiprophos methyl, 2-aminonicotinic acid; 2-chloronicotinic acid; 6-chloronicotinic acid; 2-hydroxynicotinic acid; 6-hydroxynicotinic acid; 3-hydroxypicolinic acid; Benzotriazole; 2,2xe2x80x2-dipyridil; 2,4-pyridine dicarboxylic acid monohydrate; 2-hydroxypyridine; 2,3-dihydroxypyridine; 2,4-dihydroxypyrimidine-5-carboxylic acid; 2,4-dihydroxypyrimidine-5-carboxylic acid hydrate; dinitroaniline, phosphoric amide, 2-hydroxypirimidine hydrate; 2,4,5-trihydroxypyrimidine; 2,4,6-trichloropyrimidine; 2-hydroxy4-methyl pyrimidine hydrochloride; 4-hydroxypyrazolo-3,4, d-pyrimidine; quinaldic acid; violuric acid monohydrate; thymine; xanthine; salicylic acid; sodium salicylate; salicyl aldehyde; salicyl hydrazide; 3-chlorosalicylic acid; fusaric acid; picolinic acid; butanediene monoxime; di-2-pyridyl ketone; salicin; 2,2xe2x80x2-dipyridil amine; 2,3,5-triiodobenzoic; 2-hydroxy pyridine-N-oxide; 2-hydroxy-3-nitropyridine; benzotriazole carboxylic acid; salicyl aldoxime; glycine; D L-histidine; penicillamine; 4-chlorosalicylic acid; 6-aminonicotinic acid; 2,3,5,6-tetrachloride 4-pyridine carboxylic acid; alpha benzoin oxime; 2,3-butadiene dioxime; isonicotinic hydrazide; cupferron; ethyl xanthic acid; 3-hydroxy benzyl alcohol; salicyl amide; salicyl anhydride; salicyl hydroxamic acid; methyl picolinic acid; 2-chloro pyridine; 2,6-pyridine carboxylic acid; 2,3-pyridine dicarboxylic acid; 2,5-pyridine dicarboxylic acid; Monsanto pyridones sold under the trade names Fenridazon and Genesis; pichloram; ammonium thiocyanate; amiben; diethyl dithiocarbamate; glyphosate; anthranilic acid; thiourea; 2,4-diclorophenoxyacetic acid; 4-chloro anisole; 2,3-dichloroanisole; 2-(2,4)-dichlorophenoxy propionic acid; 2-(4-chlorophenoxy)-2-methyl propionic acid; 2-(para-chloro phenoxy) isobutyric acid and xcex1,xcex2-dichlorobutyric acid.
The presently preferred sporophytic development inducers are: 2-hydroxynicotinic acid (2-HNA); 2-chloroethyl-phosphonic acid (having the commercial name of Ethrel) available from Sigma Chemical Co., PO Box 14508, St. Louis, Mo. 63178-9916; violuric acid monohydrate, 2-chloronicotinic acid and 2-hydroxyproline. The presently most preferred sporophytic development inducers are 2-hydroxynicotinic acid (2-HNA) and 2-chloroethyl-phosphonic acid. In the practice of the invention, a sufficient amount of the sporophytic development inducer is employed to effect a measurable induction of sporophytic development. The presently preferred concentration range of sporophytic development inducer is from about 0.001 mg/l to about 1000 mg/l.
While not wishing to be bound to a particular theory explaining the method of action of the sporophytic development inducers useful in the practice of the present invention, the presently preferred, representative sporophytic development inducers have some metal chelation ability. In particular, the foregoing, representative sporophytic development inducers can chelate Cu, Mg, Fe and Zn ions. Copper is essential to pollen fertility (Scharrer, K., and Schaumlaufel, E., Z. Pflanz. Dung. Bodenk, 89:1-17 (1960); see also, Tomasik, P. and Ratajewicz, Z., In: Newkome, G. R., and Strekowski, L., (eds.) Chapter 3, Pyridine-metal complexes, pp. 186-409 (1986)).
Preferably, the microspores are also contacted with an amount of an auxin effective to maintain callus development. Representative examples of auxins useful in the practice of the present invention include, but are not limited to: 2,4-dichlorophenoxyacetic acid (2,4-D), as well as related auxins (e.g., indoleacetic acid (IAA), indolebutyric acid (IBA), naphthalene acetic acid (NAA), analogues and/or salts of 2,4-D). The presently preferred concentration range for auxin is from about 0.1 mg/l to about 25 mg/l, more preferably from about 0.2 mg/l to about 10 mg/l, most preferably from about 0.5 mg/l to about 4.0 mg/l. The microspores are also preferably contacted with an amount of a cell spindle inhibiting agent effective to double the chromosome number in a measurable number of microspores. Cell spindle inhibiting agents (tubulin inhibitors) are auxin-like herbicides of which the presently most preferred is Pronamide (3,5-Dichloro [N-1, 1-Dimethyl-2-propynil] benzamide) sold by Chem Service Inc., PO Box 599, West Chester Pa. 19381-0599. The presently preferred concentration range for cell spindle inhibiting agents is from about 1.0 xcexcM to about 200 xcexcM.
Optionally microspores treated in accordance with the present invention can be contacted with an amount of a cytokinin effective to improve callus quality, in particular to enhance the ability of callus tissue to grow and to increase the size to which callus tissue develops. Representative examples of cytokinins useful in the practice of the present invention include, but are not limited to: kinetin, benzaminopurine (BAP) and zeatin. Kinetin is the presently preferred cytokinin. Additionally, water in which peeled Solanum tuberosum potatoes have been boiled contains significant amounts of cytokinin(s) which can be utilized in the practice of the present invention. The presently preferred concentration range for kinetin, zeatin and BAP is from about 0.1 mg/l to about 10 mg/l, more preferably from about 0.2 mg/l to about 4.0 mg/l, most preferably from about 0.5 mg/l to about 2.0 mg/l.
In yet another embodiment of the present invention, the microspores can be contacted with an amount of a gibberellin effective to enhance cell expansion. The presently preferred concentration range for gibberellin is from about 0.01 mg/l to about 20 mg/l, more preferably from about 0.2 mg/l to about 4.0 mg/l.
The sporophytic development inducer and auxin interact with the aforedescribed temperature and/or nutrient stress treatments to enhance the induction of androgenic microspores. In addition, the sporophytic development inducer and auxin treatments contribute to the completion of androgenesis leading to the eventual formation of mature embryoids which, upon transfer to semi-solid medium, regenerate into green or doubled haploid plants. Obtaining enlarged microspores with a fibrillar cytoplasmic structure is a pre-requisite for embryogenesis, but only those that proceed to proembryoids will eventually develop into mature embryoids, which are then able to regenerate to produce plants. Further, it will be understood that the sporophytic development inducers, auxins, cytokinins, gibberellins and temperature and nutrient stresses described herein act synergistically to produce embryoids from microspores.
An alternative to the use of whole tillers (when wheat is being treated in accordance with the methods of the present invention) is to apply a sporophytic development inducer, and preferably at least one auxin and/or cell spindle inhibiting agent, and optionally a cytokinin and/or a gibberellin, to at least one flag leaf of a spike. The flag leaf painting technique is especially suited for applications to tillers with young spikes bearing developing microspores nearer the stage of meiosis. In the presently preferred flag 4eaf painting method, a solution containing a sporophytic development inducer, and preferably at least one auxin and/or cell spindle inhibiting agent, and optionally a cytokinin and/or a gibberellin, is applied to the flag leaf of the spikes, using photographic tongs with a piece of synthetic sponge attached to each fork. The sponge pieces are dipped in the solution containing a sporophytic development inducer, and preferably an auxin and/or cell spindle inhibiting agent, and optionally a cytokinin and/or a gibberellin. For treatment, the tongs are placed such that the flag leaf is between, and in contact with, the two sponge pieces, allowing both sides of the flag leaf to be treated. The leaf is held at its tip by a finger, and is treated twice, once on the down stroke, and once lightly on the up stroke to remove excess chemical solution. The treated tillers are left on the plants until the microspores are judged to be near the late uninucleate stage of development. Then the tillers are removed, and placed in sterile distilled water, and subjected to the aforedescribed temperature and, optionally, nutrient stress treatments. The presently preferred working concentration ranges for the solution(s) of auxin, cell spindle inhibiting agent, sporophytic development inducer, cytokinin and gibberellin are as set forth above.
Treated tillers are macerated, preferably by blending in a Mini-Waring blender, and the liberated microspores are isolated by density gradient centrifugation, preferably utilizing a density gradient constructed from 0.3 M mannitol over a 0.58 M solution of maltose.
In an alternative embodiment of the present invention, microspores are first isolated, preferably by the foregoing density centrifugation procedure. The isolated microspores are placed in water and are then subjected to temperature and, optionally, nutrient stress, in the presence of a sporophytic development inducer, preferably in the presence of a sporophytic development inducer and an auxin and/or a cell spindle inhibiting agent, and optionally in the presence of a cytokinin and/or a gibberellin. The preferred temperature range and duration of temperature stress treatment, and the preferred concentrations of auxin, sporophytic development inducer, cytokinin and gibberellin, are as set forth above.
The isolated, treated microspores are then cultured in the presence of at least one live plant ovary or in the presence of an aliquot of plant ovary conditioned medium. Presently preferred sources of ovaries are wheat cultivars Pavon 76 and Chris. It has been found that the plant ovaries do not have to be from the same variety (genotype) or species as the plant from which the embryoids are generated. Thus, for example, ovaries from Igri winter barley, or NPB96001 oats, will stimulate the development of wheat embryoids.
Typically 3 to 6 live plant ovaries will be added to a 60 mm diameter Petri dish containing approximately 5 ml of embryoid culture medium. The presently preferred embryoid culture medium is NPB 99, the composition of which is set forth in Example 6 herein. Preferably the embryoid culture medium osmolarity should be around 300 mOsmo.
Alternatively, an aliquot of plant ovary conditioned medium can be used instead of live plant ovaries, as exemplified in Example 7 herein. It is understood that the use of whole, live ovaries together with an aliquot of plant ovary conditioned medium is within the scope of the present invention.
Embryoids develop from treated microspores, in the presence of at least one plant ovary (or in the presence of an aliquot of plant ovary conditioned medium), usually after 3-4 weeks. The developed embryoids are transferred to a regeneration medium, such as media 190-2 and 190-2(b) set forth in Example 6, and the remaining, less-developed embryoids are further incubated with fresh ovaries until mature embryoids form. The presently most preferred regeneration medium is 190-2(b).
Microspores (preferably uninucleate microspores) treated in accordance with the methods of the present invention can optionally be genetically transformed by any art-recognized means in order to produce plants that express one or more desirable proteins. Examples of techniques for introducing a gene, cDNA, or other nucleic acid molecule into microspores include: transformation by means of Agrobacterium tumifaciens; electroporation-facilitated DNA uptake in which an electrical pulse transiently permeabilizes cell membranes, permitting the uptake of a variety of biological molecules, including recombinant DNA, by microspores; microinjection of nucleic acid molecules directly into microspores; treatment of microspores with polyethylene glycol; and bombardment of cells with DNA-laden microprojectiles which are propelled by explosive force or compressed gas to penetrate the microspore and enter the cell nucleus.
An example of a microspore transformation technique that utilizes Agrobacterium tumifaciens and is broadly applicable to numerous plant species is disclosed in European Patent Application EP 0 737 748 A1. Isolated microspores are cocultivated with Agrobacterium containing a Ti plasmid including a transgene (within the transfer DNA of the Ti plasmid) that is to be transferred and stably integrated into the microspore genome. Cellulytic enzymes (such as cellulase, hemicellulase and pectinase) are added during the cocultivation step and serve to permeabilize the microspore cell wall. The transfer DNA (T DNA) is transferred from the Agrobacterium cells to the microspores where it is inserted into the microspore genome thereby generating stably genetically transformed microspores. Thereafter, the treated microspores are washed with a mucolytic enzyme (such as lysozyme). Whole plants can then be regenerated from the genetically transformed microspores in accordance with the present invention. Other workers have reported the use of Agrobacterium to successfully transform microspores from Brassica (Pechan P. M., Plant Cell Rep. 8:387-390 (1989); Swanson E. B. and Erickson L. R., Theor. Appl. Genet. 78:831-835 (1989)).
An example of electroporation-facilitated permeabilization of microspores is reported in Joersbo et al., Plant Cell, Tissue and Organ Culture 23:125-129 (1990). Joersbo et al. report the transient electropermeabilization of barley microspores to the dye propidium iodide by delivering rectangular electrical pulses to microspores in a chamber with cylindrical coaxial electrodes at a distance of 1 mm. The electroporation treatment had limited deleterious effect on the microspores which could be cultured to produce green plants. Similarly, Fennell and Hauptmann (Plant Cell Reports 11:567-570 (1992)) reported the electroporation-mediated delivery of plasmid DNA into maize microspores, and also reported the polyethylene glycol (PEG)-mediated delivery of plasmid DNA into maize microspores.
A presently preferred method for stably genetically transforming microspores is biolistic transformation whereby microspores are bombarded with DNA-laden microprojectiles which are propelled by explosive force or compressed gas to penetrate the microspore. Yao et al. (Genome 40(4):570-581 (1997)) report the production of transgenic barley plants by direct delivery of plasmid DNA into isolated microspores using high velocity microprojectiles. The plasmid used to transform the microspores contained a bar gene, under the control of a maize ubiquitin promoter, that conferred resistance to the herbicide bialaphos. Thus, genetically transformed microspores or embryoids could be selected based on their resistance to bialaphos present in the culture medium. Similarly, Jahne et al. (Theor. AppL Genet. 89:525-533 (1994)) also report the production of transgenic barley plants by direct delivery of plasmid DNA into isolated microspores using high velocity gold microprojectiles. Again, genetically transformed microspores or microspore-derived calli were selected based on their resistance to bialaphos present in the culture medium. Fukuoka et al. (Plant Cell Reports 17:323-328 (1998)) report the production of transgenic rapeseed plants by direct delivery of plasmid DNA into isolated microspores using high velocity microprojectiles. Transformed embryos derived from the microprojectile bombarded microspores were identified by expression of a firefly luciferase gene. Harwood et al. (Eupyhca 85:113-118 (1995)) disclose the use of the PDS1000 He particle delivery system to genetically transform barley microspores. The gus reporter gene was used to demonstrate both transient and stable transformation events. Additional examples of microspore transformation techniques are set forth in In Vitro Haploid Production in Higher Plants, Chapt. 2, Jain et al. (eds.), Kluwer Academic Publishers (1996). The aforementioned publications disclosing microspore transformation techniques are incorporated herein by reference, and minor variations make these technologies applicable to a broad range of plant species.
In each of the foregoing transformation techniques, DNA from a plasmid is genetically engineered such that it contains not only the gene of interest, but also selectable and screenable marker genes. A selectable marker gene is used to select only those microspores that have integrated copies of the plasmid (the construction is such that the gene of interest and the selectable and screenable genes are transferred as a unit). The screenable gene provides another check for the successful culturing of only those microspores or microspore-derived embryoids carrying the gene(s) of interest. A commonly used selectable marker gene is neomycin phosphotransferase II (NPT II). This gene conveys resistance to kanamycin, a compound that can be added directly to the growth media on which the microspores grow. Plant cells are normally susceptible to kanamycin and, as a result, die. The presence of the NPT II gene overcomes the effects of the kanamycin and each cell with this gene remains viable. As noted above, another selectable marker gene is a gene, such as the bar gene, which confers resistance to a herbicide. A screenable gene commonly used is the xcex2-glucuronidase gene (GUS). The presence of this gene is characterized using a histochemical reaction in which a sample of putatively transformed cells is treated with a GUS assay solution. After an appropriate incubation, the cells containing the GUS gene turn blue. Preferably, the plasmid will contain both selectable and screenable marker genes.
Plants produced in accordance with the methods of the present invention can be doubled haploids, the chromosome number of which doubled during the androgenesis induction phase of development of whole plants from microspores. Additionally, plants produced in accordance with the methods of the present invention can be haploids, the chromosome number of which can subsequently be doubled by treatment with an agent such as colchicine.
In the experience of the inventors, the methods of the present invention are at least approximately 500-fold more efficient than other methods for producing plants from microspores, efficiency being measured as the percentage of microspores that yield green plants.